Servo follower proportional control valves are well known in the art and generally described in the literature. Proportional electrohydraulic control valves have become very important in replacing the lever manual controls in tractors, backhoes harvesting equipment, utility trucks and other types of hydraulic systems which are known to be operated by a joy stick. These types of controls have become more sophisticated by the use of electronic process controllers which, for example, can be located in the cab of an agricultural vehicle to properly control tile complex operation of towed implements or other attachments operating in the back of a utility truck and controlling electrohydraulic valves at ground level.
Another example in which proportional control is extremely important and continuous is with construction equipment used to level the grade of a road relative to laser beams. In this situation, precise control is required in view of the heavy machinery and the need for accuracy. Also, accurate electrical control of the hydraulic system is critical when operating either an open loop without feedback of the road surface or a closed loop with feedback.
With the above examples in mind, it is easy to see that the reliability of a hydraulic proportional control system for heavy construction or agricultural equipment is critical in view of not only the cost of the equipment but also the safety of the personnel. In this regard, prior hydraulic proportional control systems leave much to be desired when comparing their cost and complexity to the degree that they achieve reliability and effective proportional control.
Another problem with prior proportional control systems arises when attempting to maintain a weight above ground, as for example by a backhoe. The prior equipment has had the problem that spool leakage would permit the weight in the backhoe to slowly drift down. Mechanically operated proportional control valves have used cross check valves in order to maintain the heavy weight above ground. However, with prior control valve systems, when it was desired to release the weight, the weight would bounce because the pump pressure would have to come up to a pressure high enough to unlock the check valve and thus release the cylinder. Such a bounce is even more of a problem in electrically operated proportional control valve systems.
And once the check valve is unlocked, a related problem arises when the requirement is to lower the heavy load quickly and smoothly. With prior hydraulic systems, the problem is that the load would effectively run away, and the hydraulic driving pressure would approach zero. Without pressure, the cross check valves would not operate or would actually close rather than open, and the system would stop until pressure again increased enough to open the check valve. At this point, the system would resume lowering the load until the zero pressure condition occurred again. This improper operation would repeat until the load finally reached the ground. Therefore, an operator would have to lower the load more slowly than desired in order to prevent this jerky movement on the way doom. The cause of this improper operation is that the hydraulic pump could not supply pressurized fluid fast enough to keep the cross check valves open. Even with anti-cavitation devices, the hydraulic flow would be provided but at a very low pressure.